My husband made me one of these, and it it the handiest think I have. It folds flat when you don't need it,saving you room
Select plywood the same thickness as sill;cut it as wide as the sill and 12 inches deep. Attach bottom of shelf to bottom of sill with 3 hinges;1 at the center , the others near the sides.
Cut 2 right triangles from wood thin enough to lie flat beneath shelf;trim tips. Brackets should extend 3/4 of the way across shelf and 3/4 down apron.Hinge brackets to apron.Paint before installing.